MINOR: cleanup build warnings

[mjsax]

Reviewers: Chia-Ping Tsai chia7712@gmail.com

[chia7712]

> Task :coordinator-common:compileTestJava
Note: /home/chia7712/project/kafka/coordinator-common/src/test/java/org/apache/kafka/coordinator/common/runtime/CoordinatorRuntimeTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

> Task :server:compileTestJava
Note: /home/chia7712/project/kafka/server/src/test/java/org/apache/kafka/server/KRaftClusterTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

it seems there are some remaining warnings. @mjsax do you plan to address them in this PR?